1. Field of the Invention
The present invention relates to control of wire electrical discharge machines and, more particularly, to a controller and a control method by which increased machining efficiency is achieved.
2. Description of the Prior Art
In wire electrical discharge machining, corners and other parts of a workpiece at which a wire tends to break require additional corner control and other processing such as, for example, lowering feed speed and machining current in advance. To cope with this problem, a controller for a wire electrical discharge machine has been proposed that detects an electrical discharge machining rate and controls machining speed, machining energy and flow of machining fluid accordingly to the change in the electrical discharge machining rate to prevent wire breakage and increase the machining accuracy at corners (see patent document 1 below).
The above-described controller achieves stable machining in cutting from workpiece ends and from machining start holes.
To stabilize the machining process at the start of machining, technology for switching to machining conditions different from the normal conditions from the point where the machining starts until machining proceeds a fixed distance has also been proposed (see patent documents 2 to 6 below).
Of the above technologies, patent document 2 discloses control in which set feed speed at the start of machining is reduced to 70% of set feed speed under normal machining conditions and the quiescent time is extended to 180% of the quiescent time under the normal machining conditions, and normal machining conditions are resumed gradually in a series of steps while machining proceeds to a fixed distance from the machining start point. In another type of control, disclosed in patent document 3, a switchover to weakened machining conditions is made at the start of machining, and the pressure and machining fluid flow are measured, and normal machining conditions are resumed when the measurements return to appropriate values, or when machining proceeds to a fixed distance if the measurements do not return to the appropriate values. In another type of control, disclosed in patent document 4, the workpiece end including the start point is detected by detecting a voltage change, the on- and quiescent times over a fixed distance from that point are controlled, and the machining fluid is switched. In another type of control, disclosed in patent document 5, the machining conditions are switched over to weakened machining conditions for a fixed distance from the machining start point. In the control method disclosed in patent document 6, machining conditions weaker than normal machining conditions are set at the start of machining, and the machining conditions are changed to the normal machining conditions in a continuous manner as machining proceeds.
Patent document 7 discloses a method for preventing power supply burnout by switching over to conditions weaker than usual upon detection of a short circuit between the workpiece and the electrode, to avoid the risk of fire of or damage to devices due to repeated short circuits during machining, which lead to abnormally high temperature rises in the devices in the machining power supply, and after it is recognized that the machining process has stabilized, normal conditions are restored and machining is continued.                Patent document 1: Japanese Patent Application Laid-Open No. 2002-254250        Patent document 2: Japanese Patent Application Laid-Open No. H01-264718        Patent document 3: Japanese Patent Application Laid-Open No. H05-104330        Patent document 4: Japanese Patent Application Laid-Open No. H07-266139        Patent document 5: Japanese Patent Application Laid-Open No. H04-201120        Patent document 6: Japanese Patent Application Laid-Open No. H05-111822.        Patent document 7: Japanese Patent Application Laid-Open No. H06-277949        
It has been confirmed that when the nozzle is in tight contact with a machining start hole and the hole diameter is smaller than the nozzle diameter, short circuits occur repeatedly near the hole and machining may become unstable. Possible causes of this problem are that if the nozzle covers the hole completely, it prevents proper discharging of the machining fluid ejected from the nozzle, so that metal particles and bubbles are not discharged away properly, and that turbulence of the machining fluid further amplifies wire vibration. This problem continues until the nozzle moves off the start hole so that the machining fluid can be adequately discharged.
When a technology such as that disclosed in patent documents 2 to 6 above is used to switch over to machining conditions different from normal conditions for a fixed distance from the machining start point as a means for preventing repetitive short circuits near the machining start hole, the time taken for machining is prolonged and machining efficiency is lowered.
In patent document 2, for example, performance is determined by a combination of the control distance and the initial levels of the speed and quiescent time, so these three factors need to be set individually and precisely to achieve high efficiency for various types of machining, requiring significant extra work. If large safety margins are added to these factors to eliminate the extra work, the machining efficiency is lowered. Furthermore, machining is always performed, under weakened conditions regardless of whether control dependent on the machining state is necessary, also lowering the machining efficiency.
In patent document 3, preparing cut-starting conditions for various types of machining requires extra work. If large margins are set to eliminate the extra work, the machining efficiency is lowered. Although the efficiency is increased by observing the machining fluid pressure and flow rate, this does not directly lead to stable machining. Machining still always starts under weakened conditions regardless of whether control is necessary, and the above problem remains.
A possible problem in patent document 4 is that control is not activated until a voltage change is detected during machining, without regard to the machining start point, as a result, the groove width may change during the machining process. Furthermore, because control is not activated until a voltage change is detected, machining is performed under intensive conditions in the normal state until the voltage change is detected. Considering the problems that occur at the machining start point and at the ends of workpieces, there is a risk that the wire may break immediately after a discharge, that is, before an abnormal discharge is detected due to a voltage change. Accordingly, conditions for stable machining need to be set in advance with regard to a machining start point and ends of workpieces to avoid wire breakage.
There is also a problem that preparation of conditions for stable machining involves extra work. Although enabling and disabling of this function is switched by a program command or on-screen setting, this still requires extra work by an operator. It is also difficult to correctly determine whether such control is necessary or not.
In patent document 5, extra work is necessary for preparing cut-starting conditions for various types of machining. If large safety margins are set in advance to eliminate an extra work, machining efficiency is lowered. Weakened machining conditions are always used for machining, regardless of whether such control is required or not, also resulting in poor machining efficiency.
Patent document 6 has the same problem as patent document 5, since extra work is necessary for preparing cut-starting conditions for various types of machining, and if large safety margins are set in advance to eliminate the extra work, then machining efficiency is lowered.
In patent document 7, the object is to prevent the power supply from being burnt out by repeated short circuits during continuous machining. If a short circuit is detected in the middle of machining, control is initiated even at a point other than the machining start point, as a result, a groove width may be changed during machining. Therefore, patent document 7 does not relate to a technology that solves the problem of repeatedly occurring short circuits at a machining start point.
If the technology disclosed in patent document 7 is applied at the machining start point, machining tends to destabilize for a certain distance from the ends of a workpiece or a machining start point, with the result that if conditions are restored normal because machining has once temporarily stabilized, then machining may destabilize again. Accordingly, the desired effects of the present invention cannot be expected. Another problem is that an extra work is necessary for preparation of stable conditions to cope with various types of machining.
Patent document 7 also has a problem similar to that in patent document 4. Initiation of control by detecting a short circuit means that machining is performed under intensive conditions in the normal state until a short circuit is detected. Considering problems occurring at the machining start point and the ends of the workpiece, wire may break immediately after a discharge, that is, before detecting an abnormal discharge due to short circuits. For this reason, conditions for stable machining need to be set in advance for a machining start point and ends of workpieces to prevent wire breakage.
As described above, the object of patent document 7 is to prevent the power supply from being burnt out, so that it is not reasonable to apply the technology disclosed in patent document 7 to machining start points.
Another problem is that extra works are necessary for preparation of stable conditions to cope with various types of machining.
As in the case of patent documents 2 to 6 above, which show carrying out machining at a machining start point, machining is always performed in an initial state, regardless of whether or not machining will be stable when started under normal conditions. Operations carried out from the machining start point in these control methods can generally be expected to produce results in high-speed machining and other types of machining that use energy and machining fluid under intensive conditions. However, when the usage of energy and machining fluid is not so intensive, as in the case of a primary machining in precision finish machining, stable machining is often realized even if such controls are not carried out. In such cases, these control methods work disadvantageously and lower the machining efficiency by requiring extra machining time.
FIGS. 9A and 9B illustrate how machining proceeds when no short circuit occurs within a predetermined distance from a machining start point. The “predetermined distance” is referred to as a range within which machining is initially unstable. In these figures, time required for machining to proceed the predetermined distance at a set normal feed speed v1 is T10.
FIGS. 10A and 10B illustrate how machining proceeds when short circuits occur within a predetermined distance from a machining start point. If a short circuit occurs between a wire electrode and workpiece, power supply is interrupted and discharge is restarted. Therefore, if short circuits occur repeatedly under a set normal feed speed v1, machining stops every time a short circuit occurs, with the result that the time T11 required for machining to proceed the predetermined distance becomes longer than the time T10 in FIG. 9A.
FIGS. 11A and 11B illustrate machining in which the speed setting is changed when machining proceeds to a predetermined distance from the machining start point. As described in the above patent documents, to suppress short circuits, a feed speed is set to v2, which is lower than the normal feed speed v1, until machining proceeds the predetermined distance from a machining start point. The feed speed v2 is lower than the normal feed speed v1, so the time T12 required for machining to proceed the predetermined distance is longer than the time T10 in FIG. 9A.
It is very difficult to determine whether or not such type of control is required for various types of machining, furthermore, in case where such control is required, it takes considerable time to prepare appropriate initial machining conditions. For saving time, it is necessary to set a large safety margins to cope with various types of machining, thereby lowering the machining efficiency.